Environmental Sensors and Subsystems
A Scalable, Modular, Multi-Stage, Peristaltic, Electrostatic Gas Micro-Pump
Ali Besharatian, Karthik Kumar, Rebecca L. Peterson, Luis P. Bernal, and Khalil Najafi
Conceptual illustration of the smallest pumping unit, consisting of two stages, showing the detailed layout of each chamber and fluidic paths (left), and photo of a fabricated 24-stage device after packaging, on a U.S. penny (right).
Gas micropumps are needed in many emerging applications, including gas chromatography, resonant/IR sensors, atomic clocks, and mass spectrometers. High pressure and flow are important requirements, which in turn require large-stroke and/or high-frequency actuators with low power consumption and small size. Previous gas micropumps exhibit limited capabilities due to use of bulky actuators, often were slow or power/size inefficient, and lacked scaling/integration capabilities. Our group introduced the first electrostatic peristaltic gas micropump, which utilized fluidic resonance and a multi-stage configuration to achieve the highest recorded flow and pressure in a low-power and small-volume system. However, it had inherent limitations in scalability, sealing and yield due to challenging alignment and bonding.
This project seeks to develop a MEMS pump that can be used as the roughing pump in a three-part micro-scale vacuum system. The new pump utilizes the same operating principle as previously reported, but with major modifications in device architecture, totally new fabrication technology and modular assembly/packaging. The modular fabrication technology has a final process yield of 90% with a high throughput and high control over critical design parameters (<5% error). Moreover, the device total size is 60% smaller than the old design, due to the use of a novel honeycomb planar architecture. Under preliminary testing, the microfabricated 24-stage pump successfully produced a flow rate of 0.36 sccm and a pressure accumulation of ~500 Pa at 22 kHz. This work is supported by the DARPA CSVMP program under grant #W31P4Q-09-1-0011.
A Microscale Gas Chromatograph for High-Speed Determinations of Explosive Marker Compounds
Gustavo Serrano, Lindsay Amos, Hungwei Chang, Will Collin, Nicolas Nuñovero, and Edward T. Zellers
This project demonstrates the first fully integrated, fieldable, gas chromatographic microanalytical system (µGC) for near-real-time determinations of trace-level vapor concentrations of marker compounds of explosives. This µGC, dubbed INTREPID, will be used for airport screening applications to protect workers and the public from terrorist threats. A top view of the current field prototype is shown above. It uses an adsorbent-packed, deep-reactive-ion-etched (DRIE) Si/glass microfocuser, a wall-coated DRIE-Si/glass microcolumn, and an integrated array of 4 chemiresistors coated with functionalized thiolate-monolayer-protected gold nanoparticle (MPN) interface layers. The four MPNs are: n-octanethiol (C8), 6-phenoxyhexane-1-thiol (OPH), 4-(phenylethynyl)-benzenethiol (DPA), methyl-6-mercaptohexanoate (HME), each of which yields a unique response to eluting vapors. A high-volume sampler is connected upstream to reduce analysis time and detection limits. Commercial valves and mini-pumps are used. All functions are set and automatically sequenced by a laptop computer, which runs routines written in-house in LabView. Laboratory testing of the INTREPID field prototype was performed with the following explosive markers: 2,4- dinitrotoluene (2,4- DNT) and 2,3-dimethyl-2,3-dinitrobutane (DMNB, an explosive taggant). Calculated limits of detection are 2.2 and 0.85 ng, corresponding to 0.31 and 0.11 ppb, for DMNB and 2,4-DNT, respectively (1-L sample). In the analysis of the mixture of the two markers and 19 other compounds, all targets were completely resolved from the interferences and a complete analysis required just 2 min as shown by the chromatogram above. This work is funded by the U. S. Department of Homeland Security, Science and Technology Directorate.
Multi-Transducer Arrays Using Nanoparticle Interface Layers for Vapor Discrimination
Lindsay K. Wright, Kee Scholten, and Edward T. Zellers
PCA plots for the (a) 4-sensor TSMR array, (b) 4-sensor CR array, and (c) the best performing sensor array: an MT array composed of 3 TSMR sensors and 1 CR sensor, for the 5 vapors tested. Data points are plotted for Monte-Carlo generated synthetic responses with ε =1% induced error, while ellipses represent the 95% confidence interval for data points distributed with ε =5% error.
This project explores the development of multi-transducer (MT) arrays as detectors for microscale gas chromatographs (µGC). To date, most sensor arrays are composed of a set of single transducers (ST) coated with different sorptive interface layers to impart selectivity. However, such devices have a response diversity limited by the range of vapor-film interactions, which is often insufficient for accurately differentiating between vapors, especially in mixtures. MT arrays should offer greater diversity by virtue of probing multiple aspects of the vapor-interface interactions. We are studying combinations of two transducers: chemiresistors (CR), which respond to volumetric swelling and changes in the dielectric properties of the interface film, and thickness shear mode resonators (TSMR), which respond to changes in the mass of the interface. Thin films of thiolate-monolayer protected gold nanoparticles (MPN) with different thiolate functionalities have been selected as chemically selective interfaces for both transducers. In recent work we examined the responses of CRs and TSMRs coated with four different MPN films to a set of 5 vapors (toluene, TOL; n-propanol, POH; nitromethane, NME; 2-butanone, MEK; n-octane, OCT) using extended disjoint principal components regression (EDPCR) analysis. The results showed that an optimally chosen MT array of 3 or 4 sensors had a higher recognition rate than any ST array, when considering both single vapors and binary mixtures. This work was funded by the Department of Homeland Security, Science and Technology Directorate.
Kinetic Factors Affecting the Design and Performance of Micropreconcentrators for µGC
Thitiporn Sukaew and Edward T. Zellers
a) Photograph of various µPCF designs b) Enlarged photo of the larger µPCF c) Plots of bed residence time vs. breakthrough time (10%) for different vapors with capillary-style (cPCF) and microfabricated (µPCF) devices.
The goal of this project is to investigate the effects of key design and operating variables on the dynamic adsorption capacity of adsorbent-packed microfabricated preconcentrator/focusers (µPCF) used in microscale gas chromatographic analyzers (µGC). The µPCFs under consideration comprise several single-stage deep-reactive-ion-etched Si cavities packed with a few mg of graphitized carbon adsorbent materials with high specific surface areas. The modified Wheeler model, which relates thermodynamic and kinetic parameters to the vapor capture efficiency, is used to interpret the effects of changes in cavity size, flow rate, vapor concentration, and vapor properties on the breakthrough time tB and breakthrough volume VB of the µPCF. By measuring the fractional breakthrough concentration of a vapor in a test atmosphere passing through the device versus time, one can establish limits on the allowable flow rate at which a device of given dimensions will yield quantitative capture. Tests performed with four vapors spanning a vapor pressure range of 25-95 Torr revealed that immediate breakthrough occurs if the flow rate (Q) exceeds 50 mL/min for the most volatile vapor and 230 mL/min for the least volatile vapor. Testing with a similarly packed capillary style device (cPCF) suggests a small but measurable effect arising from the adsorbent-bed geometry differences between the cPCF and µPCF. Results of these tests are being used to establish constraints on the operating parameters of µGC prototypes that employ such devices, which are being developed for explosives detection, breath biomarker measurements, and indoor air quality monitoring. This project was supported by the Department of Defense Environmental Security Technology Certification Program (DoD-ESTCP).
Micro OptoFluidic Ring Resonators for Micro Gas-Chromatograph Detectors
Kee Scholten, Xudong Fan, and Edward T. Zellers
A) SEM image of a µOFRR with 100 micron inner diameter B) Diagram illustrating the basic structure and operation of the µOFRR
This project concerns the development of a new generation of vapor sensitive micro-transducers; the microfabricated optofluidic ring resonator (µOFRR) is a whispering gallery mode resonator that integrates microfluidic and sensing functions into a single structure. The device is a cylindrical tube embedded in a Si frame, with a spherical expansion section to provide lateral confinement of optical modes. Fabrication entails defining a Si mold with deep reactive ion etching then growing the SiOx µOFRR via thermal oxidation. Devices with 50-200 µm diameters and 2 µm walls have been fabricated and characterized. In experiments, light from a coherent source was coupled into the cavity evanescently; monitoring output intensity while varying wavelength produced a series of characteristic “peaks” with quality factors exceeding 104, and measurements of the free spectral range confirmed that the resonant behavior mimics that of planar ring resonators. Recently, work has begun on a microfluidic device that incorporates 250 µm diameter µOFRRs with on-chip fluidics and structures for permanently anchoring of aligned fiber optic waveguides. This design will facilitate the use of these resonators as vapor sensors for micro gas chromatographic analysis systems. This work is supported by the National Science Foundation, the Department of Homeland Security, Science and Technology Directorate, and by the National Institute of Health, Microfluidics in Biomedical Sciences Training Program.
Multivariate Curve Resolution of Co-Eluting Peaks Measured with Microsensor Array Detectors in Micro-Scale Gas Chromatographs
Sun Kyu Kim and Edward T. Zellers
Example of EFA-ALS analysis (S/N ratio=10, R=0.5, RRR=1:1). a) Chromatograms of TCE (trichloroethylene), HEP (n-heptane), and their mixture from the least sensitive sensor (HME); b) chromatograms of the mixture from all four CR sensors; c) of true (calibrated) and recovered normalized response patterns for TCE and HEP; d) fidelity (r between true and recovered patterns) of recovered patterns of TCE and HEP and confusion (rc between true pattern of one compound and recovered pattern of the other compound). Numbers on the bars in d) are the values of r (two left-most bars) or rc (two right-most bars)
A multivariate curve resolution (MCR) method that combines evolving factor analysis (EFA) with alternating least squares (ALS) is applied to partially co-eluting vapors measured with a thiolate-monolayer-protected gold nanoparticle (MPN)-coated chemiresistor array used as the detector in a microfabricated gas chromatograph (µGC). Two pairs of vapors having different array response pattern similarities (ρ) were selected as analytes: trichloroethylene (TCE) and n-heptane (HEP), which have relatively similar patterns (ρ=0.80), and cyclohexane (CHX) and n-butanol (BOH), which have relatively dissimilar patterns (ρ=0.20). Calibration curves and a response pattern library for the individual vapors were established before the analysis. Binary mixtures were generated and analyzed at various values of chromatographic resolution (R = 0.1 to 1.0) and relative concentrations corresponding to relative response ratios ranging from 1:10 to 10:1 for the least sensitive sensor in the array. EFA-ALS analysis permitted extraction and reconstitution of responses of the individual components of the binary composite peaks with high fidelity (r > 0.95 in all cases for TCE+HEP and > 0.9 in most cases with CHX+BOH). Subsequent pattern matching with response patterns in the library was successful in all cases. Quantification of recovered responses is being explored. This project was funded by the the Department of Defense, Environmental Security Technology Certification Program (DoD-ESTCP).
On Column Optical Vapor Sensors in Micro-GC Development
Maung Kyaw Khaing Oo, Karthik Reddy, Jing Liu, and Xudong (Sherman) Fan
SERS Probes : Identifying enhanced Raman signal of gas molecules by gold nanostructures (left) and FPC Sensors : Detecting interference shift signal of two reflected beams from Si substrate and polymer-air interface caused by vapor (right)
Traditional gas chromatography tandem with mass spectroscopy system exhibits excellent detection specificity and sensitivity; however, it is bulky and has high power consumption. Applications of on-site, rapid and real time vapor analysis require innovative portable micro-gas chromatography (μGC) systems, which have been under intense study in the past couple of decades. Here, we focused on development of micro-vapor detectors that need to be sensitive, fast in response, small in size, and easily integrated with other μGC components. Fabry-Pérot (FP)-based sensors are robust, and display the potential for mass production and simple integration with current μGC technology. We developed an FP sensor which has gas sensing polymer forms part of the FP cavity. A sub-nano-gram detection limit and sub-second response time were achieved, representing orders of magnitude improvement over those previously reported. However, the FP sensor alone, the signature of vapor is not identifiable. Thus we developed another Surface enhanced Raman scattering (SERS) detector. The metallic nanostructures detector offers powerful means for express identification and trace quantities detection. We demonstrated using of Au nanostructures amplified the small Raman scattering cross section of molecules toward distinguishable signal. The detection limit of 0.4 attogram of 2,4-dinitorluene vapor was achieved. It required only a minute exposure with vapor and 2 seconds detection time.
Adaptive Two-Dimensional Microgas Chromatography
Jing Liu and Xudong "Sherman" Fan
Figure 1. (A) Schematic of the proposed adaptive 2-D GC; (B) real time chromatograms of plant emitted VOCs obtained from the adaptive 2-D GC; (C) extracted 2-D chromatogram.
We proposed and investigated a novel adaptive two-dimensional (2-D) microgas chromatography system, which consists of one 1st-dimensional column, multiple parallel 2nd-dimensional columns, and a decision-making module. The decision-making module, installed between the 1st- and 2nd-dimensional columns, normally comprises an on-column nondestructive vapor detector, a flow routing system, and a computer that monitors the detection signal from the detector and sends out the trigger signal to the flow routing system. During the operation, effluents from the 1st-dimensional column are first detected by the detector and, then, depending on the signal generated by the detector, routed to one of the 2nd-dimensional columns sequentially for further separation. As compared to conventional 2-D GC systems, the proposed adaptive GC scheme has a number of unique and advantageous features. First and foremost, the multiple parallel columns are independent of each other. Therefore, their length, stationary phase, flow rate, and temperature can be optimized for best separation and maximal versatility. In addition, the adaptive GC significantly lowers the thermal modulator modulation frequency and hence power consumption. Finally, it greatly simplifies the postdata analysis process required to reconstruct the 2-D chromatogram. The analysis of 19 plant emitted volatile organic compounds were conducted by the proposed adaptive 2-D GC with dual 2nd-column. Figure 1(B) and (C) are the real time chromatograms and extracted 2-D chromatogram, respectively. One patent about the adaptive 2-D GC has been filed at University of Michigan.
Microfabricated Passive Preconcentrator/Injector for a µGC (µPPI)
Jung Hwan Seo, Jing Liu, Xudong Fan, and Katsuo Kurabayashi
(a) Optical image showing the top layer of the µPPI with diffusion channel grids and see-through image of Carbopack X beads packed underneath. (b) Optical image of the micro-heater and RTD sensor on the backside of the bottom layer of the µPPI. (c) Plots of the theoretical and experimental injection peak band profiles of toluene at the heating rate of 90 K/s.
This project aims to develop an on-chip device named the "microfabricated passive vapor preconcentrator/injector (µPPI)." The µPPI is the first microfabricated VOC preconcentrator that enables both zero-power diffusion-based passive sampling and preconcentration of VOCs (Fig. a) and sample release/injection by controlling the temperature of its integrated on-chip micro-heater (Fig. b). The µPPI achieves a high sampling rate of 9.1 mL/min by diffusion and delivers the sampled vapor to a downstream GC component by thermal desorption at a low power of ~ 1 W. The sample loss is <5 % when a carrier gas flow rate was set at 50 mL/min. To further characterize the thermal desorption/injection performance of the µPPI , we developed an analytical model accounting for heat transfer, temperature dependent desorption kinetics, and peak band broadening. This model well predicts the real-time vapor peak band profiles in good agreement with experiment using an on-column sensor (Fig. c). Thus, we demonstrated the effect of the thermal desorption characteristics of a vapor injection device on injection peak band signal tailings and intensity. With the heating power of 1.1 W, more than 90 % of toluene is released from the adsorbent of the µPPI within 3 s. Ongoing work is focused on in-depth characterization of preconcentration, desorption, and injection performance of the µPPI for moderately complex VOC mixtures. This work was funded by the NIOSH Pilot Project Research Training Program (PPRTP) and the Michigan Center for Wireless Integrated Microsystems by the Engineering Research Centers Program of the National Science Foundation.
Adaptation of a Microscale GC for VOC Determinations of Biomarkers of Exposure/Disease in Breath and Saliva
Jonathan Bryant-Genevier, Sun Kyu Kim, Nicholas Eddy, and Edward T. Zellers
This project seeks to adapt a recently developed microfabricated gas chromatograph (µGC) field prototype, SPIRON, to the analysis of volatile organic compounds (VOCs) in human breath and saliva. The first application being pursued is the analysis of diacetyl, a butter substitute and insidious lung toxicant used in the food manufacturing industry, in saliva. Key components of this research include development of a sparger to purge diacetyl from (artificial) saliva samples into the SPIRON sampling module, quantitative capture, transfer and injection of diacetyl into the separation columns, separation of diacetyl from endogenous VOCs in saliva, and detection via an array of thiolated gold nanoparticle chemiresistor sensors, with ligands n-octaniethiol (C8), 6-phenoxyhexane-1-thiol (OPH), 4-(phenylethynyl)-benzenethiol (DPA), and methyl-6-mercaptohexanoate (HME). Results, thus far, have demonstrated successful sparging of diacetyl from artificial saliva at 60 ºC within 2L at 100 mL/min, effective capture/preconcentration of diacetyl by onboard sampler/µfocuser, and separation of diacetyl from several expected interferences. The CR array can provide an LOD of 6.4 ng based on the least sensitive sensor in the array, and the response pattern for diacetyl has been determined. Modifications to the signal circuitry and data processing have significantly improved the sensor signal stability and signal to noise ratio. The image above displays a conceptual sketch of (a)the sparging apparatus, (b) the fully assembled SPIRON field prototype with sparger attachment, and (c) a chromatogram of diacetyl with interferences generated with the SPIRON prototype (peaks 1, 3-9: acetone, THF, TCE, heptane, toluene, PCE, ethyl benzene, and xylene). The second application concerns the detection of a set of presumptive breath biomarkers of tuberculosis (TB), and entails adapting the fluidic components, materials, and operating conditions of the sampling module and (micro)analytical subsystem to detect selected the VOC biomarkers in high-humidity samples. Work on this application is ongoing. These projects have been funded by a Pilot Project Grant from NIOSH administered through the Michigan Center for Occupational Health and Safety Engineering.
Comprehensive 2-D Gas Chromatography (GC×GC) using a MEMS Thermal Modulator
Dibyadeep Paul, Gustavo Serrano, Sung Jin Kim, Will Collin, Ken D. Wise, Edward T. Zellers, and Katsuo Kurabayashi
(a)Schematic of assembled device (b) GC×GC chromatogram of a 21 compound VOC mixture
Comprehensive two-dimensional gas chromatography (GC×GC) is a powerful analytical technique to separate and detect the components of complex mixtures of volatile organic compounds. A thermal modulator is placed between two separation columns in a GC×GC system, to focus and re-inject eluting mixture components into the 2nd dimension. This enhances the resolution and selectivity of the separation. As part of our efforts to develop a μGC×μGC prototype, we have designed and fabricated a two-stage, thermal modulator (μTM) using MEMS fabrication technology. The μTM is cooled to -20 °C using a solid-state thermoelectric cooling unit and heated rapidly by resistive heaters to 210 °C. Thermal crosstalk between the two stages is less than 9%. Our μTM has fast thermal response; power consumption which is two orders of magnitude less than conventional TMs(~10 W), drastically smaller size and no usage of cryogenic consumables. We obtain peak-enhancements of ~ 45-50 and full width at half height of ~ 90. To demonstrate the feasibility of μGC×μGC, we have performed GC×GC with our μTM coupled to a 1st and a 2nd dimension column. Sets of 5-7 volatile test compounds (boiling point ≤ 174 °C) are used to study the effects of the minimum (Tmin) and maximum (Tmax) modulation temperature, stage heating lag/offset (Os), modulation period (PM), and volumetric flow rate (F) on the quality of the separations with respect to several performance metrics. A structured chromatogram is obtained using 4 sets of homologous compounds. Finally, we have demonstrated successful separation and analysis of 21 compound VOC mixture using our μTM-based GC × GC setup in 185 seconds.
Microdischarge-Based Radiation Detectors Utilizing Stacked Electrode Arrays in a TO-5 Package
Christine K. Eun and Yogesh B. Gianchandani
(a) Device concept. The detector comprises a stacked arrangement of multi-electrode stainless steel elements (i.e., anode and cathode) and a glass insulator, assembled within a commercial TO-5 package base. Gamma radiation interacts with the metal layers, which releases photoelectrons into the biased gap. These charged particles trigger avalanche within the biased gap, leading to wireless signaling. (b) Photograph of assembled device.
Motivation: We have developed a wireless gas-based beta/gamma radiation detector that uses an arrayed electrode structure to demonstrate a scalable path for increasing detection efficiency. During operation, gas microdischarges, initiated by incident radiation, can transmit wideband wireless signals. Wireless-enabled radiation sensors are envisioned for use in rapidly-deployable, mobile networks.
Device Concept & Results: The detector comprises a stacked arrangement of multi-electrode stainless steel elements (i.e., anode and cathode) and a glass insulator, assembled within a commercial TO-5 package base (Fig. 1). Each electrode is an array of 3 or 4 ‘linked’ elements.The spacer maintains a 200 µm-wide gap between electrode layers. Gamma radiation interacts with the metal layers, which releases photoelectrons into the biased gap. These charged particles trigger current avalanche and wireless signaling. The components are manufactured by commercial micromachining methods. The detector diameter and height are 9 and 9.6 mm, respectively, and its weight is 1.01 g. With a 99 µCi Cs-137 source, the detector provides >78 cpm to a hardwired interface at a source-detector distance of 30.5 cm. Receiver operating characteristics (ROC) have been shown to improve with longer integration time. The estimated intrinsic detection efficiency (i.e., with the background rate subtracted) is 3.49%. Portable power modules have been developed for this effort.
Effects of Flow Rate and Temperature on MPN-coated Chemiresistor-Array Micro-GC Detectors for Explosive Marker Compounds
Lindsay K. Wright and Edward T. Zellers
Responses from one representative MPN-coated sensor (C8) to 2,4-DNT as a function of (a) temperature and (b) flow rate. Normalized response patterns for toluene at 70°C (80 hrs, 3 mL/min) demonstrating low drift (%) and good pattern stability/fidelity (r). MPNs, from left to right: C8 (blue), DPA (red), OPH (green), HME (purple).
This project investigates thiolate-monolayer protected gold nanoparticle (MPN)-coated chemiresistor (CR) arrays as detectors for explosive marker compounds in a microscale gas chromatograph (µGC). Chromatographic resolution can be enhanced by increasing the temperature of, and flow rate over, the array, but at the cost of reduced sensitivity. Characterizing these tradeoffs is necessary in order to optimize performance. Representative results are presented in the figures above. The markers 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) are natural byproducts of TNT that are found in the headspace above TNT, while the marker 2,3-dimethyl-2,3-dinitrobutane (DMNB) is a taggant added to TNT. The alkane, n-pentadecane (C15), is used here as a representative interference. Tests show that, while increasing the array temperature from 50-70°C leads to a sensitivity decrease of 2-5 fold and an increase in the limit of detection (LOD) of up to 2-fold, the chromatographic resolution between any pair of test compounds increases by 3-6 fold (3 mL/min). By varying the flow rate from 1-3 mL/min (70°C), sensitivity decreases up to 3-fold for the marker compounds, but the LOD decreases (improves) by up to 2-fold for 2,4-DNT and 2,6-DNT due to increases in peak height. For DMNB, however, peak height reaches a maximum at ~2 mL/min and then starts to gradually decline, resulting in slight increase in the LOD at 3 mL/min. The array of sensors was operated at 70°C for 80 hours (~7 hr/day for 11 days) and the sensitivities showed only minimal drift (toluene, < 2% per day) and no significant change in the array response pattern (correlation coefficient, r ≥ 0.99). These findings are being used to guide the operating conditions used in a fieldable prototype µGC (INTREPID) employing these arrays as detectors. This work was funded by the Department of Homeland Security, Science and Technology Directorate.
Inexpensive Portable Sensors Based on Analyte-Triggered Gel Formation
Jing Chen, Yash J. Adhia, and Anne J. McNeil
Analyte-Triggered Gel Formation
Analyte-triggered solution-to-gel phase transitions have the potential to be used in simple, low-cost indicators and diagnostic devices. The principle advantages of these responsive materials, compared to colorimetric and fluorescent materials, are that the yes/no signal is unambiguously detected by sight. These gelation-based sensors can be tailored to respond to a diverse array of stimuli, including heat, light, and chemical or biological analytes, and may provide a robust and inexpensive platform for practical sensing applications in both developed and developing countries.
Over the last four years, we have reported several gelation-based sensors for the detection of nitric oxide, mercury ions, triacetone triperoxide and proteases. In each example, the response is a "yes" or "no" to the presence of analyte. These sensors would be more useful if they could also determine the amount of analyte present. Therefore, our recent efforts have been focused towards the development of quantitative gelation-based sensors utilizing several different methods, including a piezoelectric sensor, a wireless magnetoelastic sensor, and microrheology. The overall strategies involve monitoring changes in viscosity during gelation and correlating this data with the analyte concentration.